Radio receiver



D. s. BOND RADIO RECEIVER ov. 25, E941.

2 SheetsfSheet Il L SQ.

Filed Feb. 2,8, 1939 Snventor 5.502205 Nov. 25, 1941. D. s. BOND RADIO RECEAIVER Filed Feb. 28, 1959 2 Sheets-Sheet 2 www Gttorneg Patented Nov. 25, 1941 RADIO RECEIVER Donald S. Bond, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application February 28, 1939, Serial No. 258,907

(cl. 25o- 11) 3 Claims.

This invention relates to radio receivers and more particularly to a radio receiver in which means are provided for discriminating between small differences in radio guiding signals.

A radio range of the double-modulation type for guiding aircraft has been described by H. Diamond in an article Applying the Visual Double-Modulation Type Radio Range to the Airways," published in the Bureau of Standard Journal of Research, vol. 4. February 1930. The signals from a range of this type indicate a desired course, which is distinguished by a region of equal signals of two different frequencies. By way of example, Diamond used 65 cps. and 86.7 cps. as the modulation frequencies. If the radio range course is to be closely followed, it is important that the receiver be adapted to respond to small differences in the relative amplitudes of the fields of different modulation frequencies.

One of the objects of the invention is to provide means whereby a radio course is indicated. Another object is to provide means for indicating the course established by a double-modulation radio transmitter. Another object is to provide means of discriminating between small differences in magnitude of the double modulation frequencies establishing a radio course. An additional object is to provide a sensitive means for indicating a radio course independently of the field strength.

The invention will be described by reference to the accompanying drawings, in which Figure 1 is a schematic circuit diagram of one embodiment of the invention, and Figure 2 is a detailed circuit diagram of an apparatus embodying the invention.

The block diagram of Fig. 1 illustrates a radio receiver I which is connected through a low pass or band pass filter 3 to a controlled amplier 5. The control is elected by an A. V. C. amplier and rectifier 'I which is connected to the controlled amplifier. The output circuit of the controlled amplifier is coupled to an audio power amplifier 9. The power amplifier is connected through a balancing attenuator I I to a pair of band pass filters I3, I5. Each of the filters is followed by a feedback audio amplier I'l, I9.

The outputs of the feedback audio amplifiers are I applied to a balanced rectifier 2|. The balanced rectier is connected to a stabilized D. C. amplier 23. A suitable indicator 25, control device, or the like, is coupled to the D. C. amplifier.

The operation of the receiver is as follows:

The double modulation carrier, which establishes the radio course, is applied through an antenna to the receiver I. The modulated carrier is demodulated. 'I'he derived currents are passed through the filter 3 which is designed to pass only the currents of the modulation frequencies. These currents f1, f2 are applied to the controlled amplifier 5 so that the resultant output is maintained at a constant average level independently of the field strength and percentage modulation. 'I'he constant level currents are impressed through the balancing attenuator II on the pair of band pass filters I3, I5. The filters are respectivelydesigned to pass only currents of frequency f1 and frequency f2, which correspond to the double modulation frequencies of the transmitter.

These currents f1, f2 are separately amplied by audio amplifiers I1, I9. Since any change in the gain of either of the audio amplifiers might alter the ratio of the currents, the gain of each amplifier I'I, I9 is made constant by a feedback circuit which is known to those skilled in the art. The amplified curents are applied to a balanced rectifier 2 I. The output currents of the balanced rectifier may be too low to operate the indicator 25. In such case the rectified output or D. C. currents may be amplified. The amplifier is preferably stabilized. If the currents f1 and f2 are equal, the amplified currents will be equal and hence the rectified currents will be equal or zero. Therefore, the indicator 25 will remain in a neutral or on course position.

In the event that the receiver departs from the radio course one of the modulation currents will increase, while the other will decrease. The average current will be held constant. The resultant amplified currents will no longer balance in the rectifier, hence the rectier output, amplified if necessary, will not be balanced, and the indicator will depart from its neutral position indicating a corresponding course departure. As the distance between the transmitter and receiver is decreased, the eld strength increases but such increase will not affect the balanced rectifier currents because the average of the two audio currents is maintained constant by the controlled amplifier. Furthermore, the arrangement described has been adjusted so perfectly that a D. C. output of to 100 volts has been obtained for 1 db. difference between the two modulation voltages f1, f2.

inasmuch as the schematic diagrams, such as Fig. 2, are so well known to those skilled in the art, it is not necessary to describe in detail the elements within the several broken line areas quencies; means for deriving from Vsaidresponsive means signal voltages corresponding to said modulation frequencies; an audio frequency amplifier for said derived signal voltages; means connected to the output of said audio frequency amplifier for varying the gain of said amplifier in inverse proportion to the average amplitude of the signal input of :said amplifier so lthat the ree sultant output is maintained at a constant average level independent of the field strength and percentage modulation yof the `course-indicating signals frequency selective .means for separating the signal output of said amplifier into two channels; and means responsive to the differential of said separated signal voltages for indicating the relative'arnplitude of said voltages.

2. A radio receiver lcomprising a radio frequency section having 'a :substantially constant 'gain characteristic rand an `audio frequency section, said audio frequency section including an amplifier; means connected to the output of said amplifier for varying the gain of said amplifier in inverse proportion to the amplitude of the signal input to said amplifier to maintain a constant average output independent of variations in the average input; frequency selective means coupled to said amplifier for separating the signal output of said amplier into two separate channels; and means responsive to the difference in amplitude of the signals in said channels.

3. A radio receiver comprising a radio fre- .quency section having a substantially constant gain characteristic and an audio frequency section, said audio frequency section including an amplifier; means connected to the output of said amplifier for varying thergain of said amplifierY in inverse proportion to the amplitude of the signal input to said amplifier to maintain the output of said amplifier constant and independent of variations in the .average input; frequency selective means coupled lto said amplifier for separating the signal output of said amplifier into two separate channels; a pair of amplifiers connected in said channels; feedback means for stabilizing the gain of each amplifier of said pair; means for deriving a voltage proportional to the difference in amplitude between the output voltages of said stabilized amplifiers; and means for indicating the amplitude of said difierential voltage.

DONALD S. BOND. 

